Starwatch: this week’s rare blue moon highlights the strange way we mark our calendar

Because we define dates based on the stars, the full moon on 31 May will be the second one of the calendar month This week’s full moon is a blue moon. The term does not describe the colour of the moon, but instead arises from the way we define our calendar in reference to the stars rather than the moon. The moon takes almost a month to circle our planet. The exact time is 29.5 days but if we were to define our calendar as 12 lunar months, the year would fall short by around 11 days. Thus, the ca